Observation of the rare decay $\eta$ $\to$ $\mu^+\mu^-$e$^+$e$^-$

TL;DR

This paper reports the first observation of the rare decay eta to mu+ mu- e+ e- by the CMS Collaboration at CERN, measuring its branching fraction with data from 2022, consistent with theoretical predictions.

Contribution

The paper presents the first experimental observation and measurement of the eta decay to mu+ mu- e+ e-, using proton-proton collision data at 13.6 TeV.

Findings

Measured branching fraction: (2.4 ± 0.8)×10^{-6}

Result is close to two orders of magnitude smaller than previous limits

Measurement aligns with recent theoretical models

Abstract

A first observation of the rare decay $\eta$ $\to$ $\mu^+\mu^-$e$^+$e$^-$ is reported by the CMS Collaboration at the CERN LHC. The result is based on a proton-proton collision data sample at $\sqrt{s}$ = 13.6 TeV corresponding to an integrated luminosity of 38.0 fb$^{-1}$, acquired in 2022 using a high-rate dimuon trigger. Using the $\eta$ $\to$ $\mu^+\mu^-\gamma$ decay channel for normalization, the branching fraction is measured to be $\mathcal{B}$($\eta$ $\to$ $\mu^+\mu^-$e$^+$e$^-$) = (2.4 $\pm$ 0.8)$\times$ 10$^{-6}$, with the uncertainty including statistical and systematic sources as well as the $\mathcal{B}$($\eta$ $\to$ $\mu^+\mu^-\gamma$) uncertainty. This result is close to two orders of magnitude smaller than the existing limit, and is consistent with recent theoretical predictions.